1. Technical Field
The invention relates generally to steering shafts, and more particularly to upper steering shafts for use in automobiles.
2. Related Art
Steering shafts for automobiles typically comprise an upper steering shaft joined to a lower steering shaft for collapsible telescoping movement therebetween. To accommodate the need for collapsible telescoping movement relative to the lower steering shaft, axially traversing splines are commonly formed in the bore of the upper steering shaft. The upper steering shaft is formed to receive a hand wheel such that rotation of the hand wheel causes the upper steering shaft and lower steering shaft to rotate about a longitudinal axis.
The upper steering shaft is commonly formed from a single piece of material, and most commonly by an extrusion process. In order to form the complex, and in some cases critical dimensions of the upper steering shaft, a push-pull extrusion process is typically used to achieve the desired form. The push-pull extrusion process can take anywhere from ten or more steps to create the complex features and dimensions of the upper steering shaft. The relatively high number of steps are required to form the complex dimensions of the upper steering shaft, including the plurality of reductions necessary to accommodate the attachment of the hand wheel to the upper steering shaft and to provide for telescoping movement of the upper steering shaft relative to a lower steering shaft. Though the push-pull extrusion process creates an effective upper steering shaft, the process can prove to be both timely and costly. Additionally, special considerations must be given to producing the wall thicknesses necessary to achieve the torsional strength requirements for the upper steering shaft.
As shown in FIG. 1, an upper steering shaft depicting prior art is shown constructed from a single piece of tubular stock material. The upper steering shaft is formed using a push-pull extrusion process. The push-pull extrusion process provides the ability to produce the plurality of reductions in diameter across the length of the shaft. The upper steering shaft has a threaded end for receiving a nut (not shown) to fasten a hand wheel to the steering shaft. Adjacent the threaded end is a hub engagement portion comprising a tapered portion and a splined segment with an annular recess therebetween. The annular recess region can provide a reduced wall thickness inherent to extruding the complex geometry in this region, thus reducing the torsional strength of the upper steering shaft. Thus, extruding a single piece of material to produce the upper steering shaft having the required torsional strength can prove challenging. Additionally, above and beyond achieving the required torsional strength parameters, the plurality of reductions in diameter across the length of the upper steering shaft provide inherent manufacturing challenges, especially if utilizing an extrusion process. Thus, forming an upper steering shaft from a single piece of tube stock, though capable of producing an effective and reliable component, has proven to be difficult, timely and costly.